Carbon tool steel strip

ABSTRACT

Provided is a carbon tool steel strip suitable for use in various spring materials, valve materials, and the like, in which press punching properties and fatigue characteristics are enhanced. A carbon tool steel strip having a thickness of 1 mm or less and a carbon tool steel composition containing 0.8-1.2% C by mass %, wherein the carbon tool steel strip has a Vickers hardness of 500-650 (Hv), and when a cross-section at the center in the sheet thickness direction of the carbon tool steel strip is viewed with the plane of observation in a direction at a right angle to a rolled surface of the carbon tool steel strip and in the length direction of the carbon tool steel strip, the area ratio of carbides having an equivalent circle diameter of at least 0.5 μm among the carbides present in the metallographic structure is 0.50-4.30%.

TECHNICAL FIELD

The present invention relates to a carbon tool steel strip suitable foruse in various springs or valve materials, such as shock absorbers orflapper valves.

BACKGROUND ART

Conventionally, a carbon tool steel strip suitable for use in springs,valves, or the like is rolled to a predetermined thickness, thenquenched and tempered to adjust target properties, and then processedinto a target shape for use by e.g. blanking.

For carbon tool steel used after quenching and tempering in this manner,JP-A-2006-63384 (Patent Literature 1), for example, proposes to improveimpact properties by controlling an amount of residual carbides in ametallographic structure of the carbon steel. The proposed high carbonsteel member has the improved impact properties by controlling a volume% (Vf) of unsolved carbides in a matrix within a range of8.5<15.3×C%−Vf<10.0.

CITATION LIST Patent Literature

Patent Literature 1: JP-A-2006-63384

SUMMARY OF INVENTION

A carbon tool steel strip used for spring materials, valve materials, orthe like is also required to have excellent blanking properties forbeing punched in a shape of a spring or a valve and high fatigueproperties in addition to impact properties. However, sufficient studieshave not been made on what metallographic structure is appropriate as acarbon tool steel strip in order to achieve both fatigue propertiesnecessary for use as a spring or a valve and blanking propertiesimportant in a manufacturing process.

It is an object of the present invention to provide a carbon tool steelstrip having improved blanking and fatigue properties and suitable foruse in various spring materials, valve materials or the like.

The present inventors have found an appropriate form of carbides in acarbon tool steel strip that can achieve both fatigue and blankingproperties in addition to improving hardness suitable for use insprings, valves or the like. Thus they arrived at the present invention.

Specifically, the present invention relates to a carbon tool steel striphaving a composition comprising 0.8 to 1.2% of carbon by mass % andhaving a thickness of not greater than 1 mm. The strip has a Vickershardness of 500 to 650 HV. When an observation plane is takenperpendicular to a rolled surface of the strip and parallel to a lengthdirection of the strip and a cross-section is observed at a centerportion in a thickness direction of the strip, an area ratio of carbideshaving an equivalent circle diameter of at least 0.5 μm among carbidespresent in a metallographic structure is 0.50 to 4.30%.

The area ratio of the carbides having an equivalent circle diameter ofat least 0.5 μm is preferably in a range of 1.50% to 4.00%. In addition,the strip preferably has a thickness of 0.1 to 0.5 mm.

The strip of the present invention can have optimum hardness as well asboth fatigue and blanking properties. The strip is particularly suitablefor various spring or valve materials having a thickness of 0.1 to 0.5mm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an electron micrograph of a cross-section of the carbon toolsteel strip according to the present invention.

FIG. 2 is a schematic diagram showing an observation plane of across-sectional metallographic structure of the strip.

DESCRIPTION OF EMBODIMENT

As described above, an important aspect of the present invention is toadjust an area ratio of carbides larger than a certain size afterquenched and tempered to a certain range. Further, while hardness of thestrip is optimized, both fatigue and blanking properties can beachieved.

Therefore, the strip according to the present invention has propertiesrequired as springs or valves at the same time. The thickness thereof islimited to not greater than 1 mm so that it can be applied to thesprings, valves or the like.

First, a composition of the carbon tool steel strip according to thepresent invention will be described. The content of each element isexpressed by mass %.

C: 0.8 to 1.2%

Carbon is a necessary element for obtaining mechanical properties suchas hardness, or adequate wear resistance and impact resistance afterquenching and tempering. In addition, carbon is necessary to adjusthardness and a form of carbides in appropriate ranges. Therefore, atleast 0.8% of carbon is necessary. However, 1.2% of carbon lowers Mspoint and causes deterioration of properties since retained austeniteand residual carbides increase. Therefore, the carbon content is 0.8 to1.2%. A preferable lower limit of carbon is more than 0.9%, and apreferable upper limit of carbon is 1.1%.

The composition of the carbon tool steel strip according to the presentinvention is not particularly limited except for the carbon content, andthe strip may have a composition of carbon tool steel defined inJIS-G-3311 (Cold rolled special steel strip). Particularly, followingcomposition is preferable.

Si: 0.1 to 0.35%

Si is added as a deoxidizing material for refining. Less than 0.1% of Sipossibly has insufficient deoxidization effect and more than 0.35% ofsilicon may deteriorate mechanical properties. Therefore, the Si contentis preferably 0.1 to 0.35%. A further preferable lower limit of Si is0.15%, and a preferable upper limit of Si is 0.30%.

Mn: 0.1 to 0.5%

Mn is effective in improving quenching properties. Less than 0.1% of Mnpossibly has insufficient improvement effect and more than 0.5% of Mnmay deteriorate toughness. Therefore, the Mn content is preferably 0.1to 0.5%. A preferable lower limit of Mn is 0.35%, and a preferable upperlimit of Mn is 0.48%.

Cr: 0.05 to 0.3%

Cr is effective in improving quenching properties. Therefore, it ispreferable to add more than 0% of Cr. However, when Cr is added morethan 0.3%, a pearlite is likely to be formed, and blanking propertiesare deteriorated. Therefore, a preferable amount of Cr is more than 0%and not more than 0.3%. In order to surely improve the quenchingproperties, a lower limit of Cr is preferably 0.05%. Since an oxide filmis likely to be formed during quenching and tempering by the addition ofCr, a preferable upper limit of Cr is 0.25%.

Elements other than the above may be Fe and impurities.

Next, an area ratio of carbides defined in the present invention will bedescribed.

In a carbon tool steel strip having the composition described above, amatrix of a metallographic structure of the strip is martensite afterquenched and tempered. When retained austenite and pearlite increases,mechanical properties are deteriorated.

According to the present invention, the area ratio of carbides having anequivalent circle diameter of not less than 0.5 μm in the metallographicstructure mainly composed of the martensite structure is defined as 0.50to 4.30%. The carbides having the equivalent circle diameter of not lessthan 0.5 μm are likely to become origins of fatigue cracks. Thus, theamount of the carbides having the equivalent circle diameter of not lessthan 0.5 μm should be as small as possible in order to improve fatigueproperties. On the other hand, when press works are conducted, thecarbides having the equivalent circle diameter of not less than 0.5 μmbecome crack origins and are effective in reducing a punching load.

Therefore, the area ratio of the carbides having the equivalent circlediameter of not less than 0.5 μm has an effect on both fatigue andblanking properties. There is a preferable range that achieves bothfatigue and blanking properties. When the area ratio of the carbideshaving the equivalent circle diameter of not less than 0.5 μm in themetallographic structure of the strip is less than 0.50%, the blankingproperties deteriorate significantly. When the area ratio is more than4.30%, the fatigue properties deteriorate significantly. Thus, accordingto the present invention, the area ratio of the carbides having theequivalent circle diameter of not less than 0.5 μm in the metallographicstructure is defined as 0.50 to 4.30%. Further preferably, the lowerlimit of the area ratio is 1.50%, and a preferable upper limit of thearea ratio is 4.00%. Although an upper limit of the carbide size is notparticularly defined, when carbides having a size of larger than 5 μmare present for example, they are likely to causes fatigue cracks andthe fatigue properties may be deteriorated significantly. Thus, theupper limit of the carbide size is preferably 5 μm in equivalent circlediameter.

For evaluating the area of the carbides, a center portion in a thicknessdirection of a quenched and tempered carbon tool steel strip is observedsuch that an observation plane is taken in a length direction as shownin FIG. 2. Since the thickness of the strip is small, observation of themetallographic structure in a vicinity of a surface of the strip isaffected by test piece preparation and thus variations in themetallographic structure are large.

A test piece for the carbide evaluation is embedded in a resin such thatan observation plane 3 is perpendicular to a rolled surface 2 andparallel to a length direction of the strip 1 as shown in FIG. 2. Thetest piece is mirror-polished and then immersed in a sodiumpicrate-alkali liquid heated at 80 to 100° C. for about 10 minutes tocolor the carbides. An area of 6000 μm² at the center portion of theobservation plane 3 is observed in a backscattered electron image with2000 magnifications using a scanning electron microscope. The evaluationis possible with image processing such that only carbides having anequivalent circle diameter of not less than 0.5 μm among the observedcarbides are identified to measure the area ratio.

The carbon tool steel strip is made to have Vickers hardness of 500 to650 HV. Sufficient spring properties can not be ensured with a hardnessof at less than 500 HV, while strength becomes too strong with ahardness of greater than 650 HV and the strip does not function as aspring or a valve. A preferable lower limit of the Vickers hardness is520 HV, and a preferable upper limit of the Vickers hardness is 625 HV.

The carbon tool steel strip described above in detail may have athickness to 0.1 to 0.5 mm for suitably used for springs or valves.

It is preferable to select quenching conditions in which a quenchingtemperature is slightly higher than that of conventional quenching and atempering temperature is also higher, in order to provide a carbide formwhere the area ratio of the carbides having the equivalent circlediameter of not less than 0.5 μm in the metallographic structure is 0.50to 4.30% according to the present invention. As specific quenchingconditions, it is preferable that the quenching temperature is 830 to940° C. and the heating and holding time is 20 to 170 seconds. When thequenching temperature is lower than 830° C., solution of carbides isinsufficient and it is difficult to obtain the above carbide form. Whenthe quenching temperature is higher than 940° C., a problem is likely tooccur in which too much carbides are solved and residual carbidesdecrease. A preferable lower limit of the quenching temperature is 850°C. and a preferable upper limit of the quenching temperature is 920° C.In addition, when the heating and holding time for the quenching isshorter than 20 seconds or longer than 170 seconds, the area ratio ofcarbides of not less than 0.5 μm is less likely to be 0.50 to 4.30%.

As a rapid cooling method for the quenching, it is preferable that thesteel strip is cooled to 200 to 350° C. in a salt bath, a molten metal,a mist or the like, and then the strip is sandwiched between twowater-cooled press plates made of Cu or cast iron to be further cooledwhile being straightening, so as to complete martensitic transformation.

In addition, it is preferable to select suitable tempering conditions toprovide appropriate hardness. Specifically, a tempering temperature ispreferably in a range of 310 to 440° C. When the tempering temperatureis lower than 310° C., the hardness increases. When the temperingtemperature is higher than 440° C., the hardness is likely to decrease.A further preferable lower limit of the tempering temperature is 350°C., and a further preferable upper limit is 400° C. In addition, thetempering time is preferably 30 to 300 seconds. When the tempering timeis shorter than 30 seconds, the hardness is likely to increase. When thetempering time is longer than 300 seconds, the hardness decreases toomuch.

In order to more surely increase fatigue strength of the carbon toolsteel strip according to the present invention, surface roughness of thestrip is made such that ten-point average roughness (Rz) defined inJIS-B-0601 is not more than 0.5 μm and arithmetic average roughness (Ra)is not more than 0.08 μm. In the ranges of the surface roughness,fatigue failure originating from surface defects such as flaws on thestrip surface can be more surely prevented. The surface roughness ispossibly different between front and back surfaces of the strip, and itis more preferable that the difference in roughness between the frontand the back surfaces is within 0.15 μm (preferably within 0.10 μm) forthe ten-point average roughness and within 0.015 μm (preferably within0.012 μm) for the arithmetic average roughness (Ra).

In order to obtain the above-described surface roughness, the front andback surfaces of the strip after quenched and tempered may be physicallyablated. For example, buffing with alumina or silica abrasive particlesmay be conducted.

EXAMPLES

The present invention will be described in more detail with thefollowing Examples.

A carbon steel raw material was melt, cast and then hot-rolled toprovide a hot-rolled material. The hot-rolled material was repeatedlycold-rolled and annealed to prepare a cold-rolled material A of a carbontool steel strip having a thickness of 0.30 mm and a cold-rolledmaterial B having a thickness of 0.20 mm.

The thickness and chemical composition of the cold-rolled materials Aand B are described in Table 1.

TABLE 1 Thickness Composition (mass %) Material (mm) C Si Mn Cr BalanceA 0.30 0.965 0.245 0.440 0.131 Fe and inevitable impurities B 0.20 0.9650.245 0.440 0.131 Fe and inevitable impurities

The cold-rolled materials A and B were quenched and tempered to producecarbon tool steel strips.

Area ratio of carbides having an equivalent circle diameter of not lessthan 0.5 μm in a metallographic structure was varied by changingquenching conditions. The quenching and tempering conditions are shownin Table 2.

For the quenching and tempering, the temperature was set as shown inTable 2, and the cold-rolled material was held in a furnace for eachtime. Rapid cooling for the quenching was conducted with the materialsandwiched between water-cooled press plates.

The tempered carbon tool steel strip was polished on front and backsurfaces of the strip by buffing with alumina abrasive particles.

TABLE 2 Sample Material Quenching conditions Tempering conditions 1 A900° C. × 60 seconds 380° C. × 90 seconds holding → Rapid coolingholding → Air cooling 2 A 900° C. × 30 seconds 380° C. × 90 secondsholding → Rapid cooling holding → Air cooling 3 A 900° C. × 15 seconds380° C. × 90 seconds holding → Rapid cooling holding → Air cooling 4 B900° C. × 40 seconds 380° C. × 60 seconds holding → Rapid coolingholding → Air cooling 5 B 900° C. × 20 seconds 380° C. × 60 secondsholding → Rapid cooling holding → Air cooling

A test piece for observing a metallographic structure was cut from theproduced carbon tool steel strip. The cut portions are shown by brokenlines in FIG. 2. Then, the test piece was embedded in a resin such thatan observation plane 3 is perpendicular to the rolled surface 2 of thestrip 1 and parallel to the length direction of the strip as shown inFIG. 2. The test piece was mirror-polished and then immersed in a sodiumpicrate-alkali liquid heated at 80 to 100° C. for about 10 minutes tocolor carbides. An area of 6000 μm² at a center portion in a thicknessdirection was observed in a backscattered electron image at 2000magnifications using a scanning electron microscope, and imageprocessing was conducted. The area ratio of carbides of not lower than0.5 μm was evaluated.

Next, five-point average Vickers hardness was measured in the vicinityof the observation portion of each sample. In addition, a test piece forevaluating fatigue properties was taken in a rolled direction.Alternating bending stress was applied and an S-N curve was produced toobtain a strength at finite life of 10⁷ times under which the stripbreaks for 10⁷ repetitions.

In addition, evaluation of blanking properties was conducted. A load wasmeasured when a test piece having 10 mm width was punched with a punchand a die of 10 mm square. The measurement results are shown in Table 3.

As the backscattered electron image with 2000 magnifications of theobservation plane of carbides, the image of Sample 2 is shown in FIG. 1.Black portions in FIG. 1 are carbides. The lateral direction in FIG. 1corresponds to the length direction of the carbon tool steel strip. Itis seen that carbides of not less than 0.5 μm are present at apreferable area ratio. In addition, as shown in FIG. 1, the maximum sizeof the carbides of Sample 2 is about 3 μm in equivalent circle diameter.The carbide maximum size of other samples of the present invention wasalso about 2 to 3 μm in equivalent circle diameter.

A test piece for observing the metallographic structure was taken from aposition similar to that of the observation plane of the carbides. Itwas embedded in a resin, mirror-polished, and then etched with nital (amixed solution of nitric acid and ethanol). Microstructure was observedby an optical microscope and it was confirmed that a matrix of themetallographic structure of the carbon tool steel strip wassubstantially martensite.

In addition, surface roughness after buffing was measured. The surfaceroughness was Rz: 0.39 μm at front surface, Rz: 0.33 μm at back surface,Ra: 0.068 μm at the front surface, and Ra: 0.061 μm at the back surface.It was confirmed that the carbon tool steel strip had smooth surfaceroughness.

TABLE 3 Strength at Area ratio of finite life of carbides of notHardness 10⁷ times Press load Sample Material less than 0.5 μm (HV)(MPa) (kN) Note 1 A 0.58 555 1150 8.39 Present invention 2 A 3.69 5661050 8.27 Present invention 3 A 4.34 547 1000 7.86 Comparative example 4B 1.60 577 1100 5.90 Present invention 5 B 2.02 588 1050 5.85 Presentinvention

It is seen from Table 3 that the carbon tool steel strips in which thearea ratio of carbides having an equivalent circle diameter of not lessthan 0.5 μm in the metallographic structure is within the range definedin the present invention have a hardness of greater than 550 HV and astrength at finite life of 10⁷ times of greater than 1000 MPa. They havea low press load of not greater than 8.5 (kN) for the materials having athickness of 0.3 mm and not greater than 6.0 kN for the materials havinga thickness of 0.2 mm.

It is seen therefrom that a carbon tool steel strip in which the arearatio of carbides having an equivalent circle diameter of not less than0.5 μm in the metallographic structure is 0.50 to 4.30% has an excellentbalance between mechanical properties.

The carbon tool steel strip according to the present invention canachieve both product properties, such as hardness and fatigueproperties, and blanking properties. Therefore, it can be expected to beapplied to springs or valves, particularly having a thickness of 0.1 to0.5 mm.

REFERENCE SIGNS LIST

1 carbon tool steel strip

2 rolled surface

3 observation plane

1. A carbon tool steel strip having a composition comprising 0.8 to 1.2%of carbon by mass % and having a thickness of not more than 1 mm,wherein the strip has a Vickers hardness of 500 to 650 HV, and whereinthe strip has an area ratio of carbides having an equivalent circlediameter of not less than 0.5 μm among carbides in a metallographicstructure is 0.50 to 4.30%, when an observation plane is takenperpendicular to a rolled surface of the strip and parallel to a lengthdirection of the strip and a cross-section is observed at a centerportion in a thickness direction of the strip in the observation plane.2. The strip according to claim 1, wherein the area ratio of thecarbides having an equivalent circle diameter of not less than 0.5 μm is1.50 to 4.00%.
 3. The strip according to claim 1, having a thickness of0.1 to 0.5 mm.
 4. The strip according to claim 2, having a thickness of0.1 to 0.5 mm.